1. Field of the Invention
The present invention relates to an exposure system and an exposure method and more particularly, to an exposure system and an exposure method that are effective for an exposure process using deep ultraviolet (Deep-UV) light and that are applicable to fabrication of semiconductor devices or the like.
2. Description of the Prior Art
To form integrated circuits on a semiconductor wafer, generally, an exposure system is used to transfer patterns of geometrical shapes formed on an exposure mask to the wafer. An example of the conventional exposure systems is shown in FIG. 1, which is a reduction step-and-repeat projection exposure system.
The conventional projection exposure system shown in FIG. 1 is equipped with a light source unit 39, a reflection mirror 40, a fly-eye lens 41, an aperture stop 42, a condenser lens 43, a reduction projection lens system 45, and a wafer stage 50.
The light source unit 39 generates exposing light and emits a beam 38 of the exposing light to the reflection mirror 40. The mirror 40 reflects the beam 38 and changes its direction toward the fly-eye lens 41. The fly-eye lens 41 receives the beam 38 of the light to minimize its illumination variation or fluctuation, making its intensity uniform over the entire beam 38 at the outlet of the lens 41.
The aperture stop 42 has a filter 42a for narrowing the beam 38 of the exposing light penetrating through the fly-eye lens 41 and for adjusting the intensity distribution within the cross-section of the beam 38. The stop 42 with the filter 42a can be turned around a vertically fixed shaft 42b so that it is on or out of the optical path of the beam 38 for replacement.
The condenser lens 43 receives the narrowed and intensity-adjusted beam 38 by the aperture stop 42 to focus it on a reticle 44 placed between the lens 43 and the projection lens system 45. The reduction projection lens system 45 projects an image of the patterns of geometrical shapes formed on the reticle 44 to a semiconductor wafer 51 placed on the wafer stage 50 with a specified demagnification ratio. These patterns define the various regions in an integrated circuit (IC) such as the implantation regions and the contact windows.
The patterns of the geometrical shapes on the reticle 44 are 2 to 10 times as large as original patterns to be transferred. In other words, the demagnification or reduction ratio of the exposure system is set as 2 to 10.
The wafer 51, which is placed on the wafer stage 50, has a plurality of image fields arranged in a matrix array. Each of the image fields is exposed to the beam 38 of the exposing light at a time. The shape and size of each image field are, for example, square and approximately 20 mm.times.20 mm, respectively.
The image fields of the wafer 51 are stepped over its surface by two-dimensional translations of the wafer 51, which is generated by movement of the wafer stage 50. Thus, all the image fields are successively exposed to the beam 38 of the exposing light, thereby transferring the patterns of the reticle 44 onto all the image fields.
FIG. 2 shows the structure of an example of the light source unit 39 shown in FIG. 1. When the g-line with a wavelength of 436 nm or the i-line with a wavelength of 365 nm is used as the exposing light, a mercury (Hg) lamp 49 is used as a source of the exposing light. A beam 47 of the g- or i-line is condensed by ellipsoidal mirrors 48 disposed near the lamp 49. The condensed beam 47 is reflected by a reflection mirror 52 and is entered a collimating lens 46. The lens 46 collimates the incident beam 47, thereby making the beam 47 parallel. The parallel beam 47 is propagated to the mirror 40 through an illumination lens system (not shown) and is used as the beam 38 of the exposing light shown in FIG. 1.
When KrF excimer light with a wavelength 248 nm or ArF excimer light with a wavelength 193 nm is used as the exposing light, a KrF or ArF excimer laser system is generally used instead of the mercury lamp 49.
The above four sorts of the exposing light are within the Deep-UV region of the electromagnetic spectrum.
The light intensity distribution of the beam 38 of the exposing light governs the resolution of the patterns of the geometrical shapes of the reticle 44. Therefore, the aperture stop 42 and/or the filter 52a thereof are necessary to be replaced according to the sorts of the patterns on the reticle 44.
On replacement of the stop 42 and/or filter 42a, the stop 42 with the filter 42a is turned around the vertical shaft 42b to be out of the optical path of the beam 38. Then, after finishing the replacement, the stop 42 and the filter 42a are rotated back around the shaft 42b to its original position again, which is on the optical path of the beam 38 under the fly-eye lens 41.
FIGS. 3A and 3B show the light intensity distribution of the beam 38 at the outlet of the fly-eye lens 41. Typically, the intensity distribution of FIG. 3A is used, which is relatively high for the central area and relatively low for the edge or periphery. This distribution is called as the "Gaussian distribution".
However, for fine or minute patterns of geometrical shapes, it is known that a uniform intensity distribution (as shown in FIG. 3B) of the beam 38 of the light improves the depth of focus (DOF) of the beam 38, because the component of the beam 38 entering obliquely the reticle 44, which corresponds to the component penetrating through the edge or periphery of the fly-eye lens 41, plays an important role. This fact was, for example, disclosed by K. Yamanaka et al, in "NA and .sigma. Optimization for High NA I-line Lithography", SPIE Vol. 1927, Optical/Laser Microlithography VI, 1993, pp. 310 to 319. This article reported the following matter:
The i-lien was used as exposing light, the numerical aperture (NA) of a reduction projection lens system was 0.6, and a coherence factor .sigma. of illumination and projection lens systems was 0.7. A lines and spaces (L/S) pattern of 0.35 .mu.m was formed on a reticle. The DOF for a uniform distribution (as shown in FIG. 3B) at the outlet of an aperture stop was more advantageous than that for the above Gaussian distribution (as shown in FIG. 3A).
The illumination of the mercury lamp 49 in the light source unit 39 tends to deteriorate with time and therefore, the lamp 49 is necessary to be replaced periodically. With the above conventional projection exposure system of FIG. 1, due to the time-dependent deterioration in illumination of the lamp 49 and/or replacement thereof, the light intensity distribution of the beam 38 of the exposing light changes, thereby causing some change in optical conditions of the exposure system.
As a result, inferior or unsatisfactory pattern transfer often occurs because of degradation in DOF and/or resolution. Also, the pattern transfer formation process cannot be optimized according to a particular type of patterns on the reticle 44.
To solve such the problem, a photographic film may be inserted within the optical path of the beam 38 to print its light spot thereon. Although this technique enables to know the intensity distribution of the beam 38, it increases the downtime for this pattern transfer process. Thus, this technique is difficult to be actually used.